Cytosine (; C) is one of the four main bases found in DNA
, along with adenine
, and thymine
in RNA). It is a pyrimidine
derivative, with a heterocyclic aromatic ring
and two substituents attached (an amine
group at position 4 and a keto
group at position 2). The nucleoside
of cytosine is cytidine
. In Watson-Crick base pair
ing, it forms three hydrogen bond
s with guanine
Cytosine was discovered and named by Albrecht Kossel
and Albert Neumann in 1894 when it was hydrolyzed from calf thymus
tissues. A structure was proposed in 1903, and was synthesized (and thus confirmed) in the laboratory in the same year.
In 1998, cytosine was used in an early demonstration of quantum information processing
when Oxford University researchers implemented the Deutsch-Jozsa algorithm
on a two qubit nuclear magnetic resonance quantum computer (NMRQC)
In March 2015, NASA scientists reported the formation of cytosine, along with uracil and thymine, from pyrimidine
under the space-like laboratory conditions, which is of interest because pyrimidine has been found in meteorites although its origin is unknown.
Cytosine can be found as part of DNA, as part of RNA, or as a part of a nucleotide
. As cytidine triphosphate
(CTP), it can act as a co-factor to enzymes, and can transfer a phosphate to convert adenosine diphosphate
(ADP) to adenosine triphosphate
In DNA and RNA, cytosine is paired with guanine
. However, it is inherently unstable, and can change into uracil
). This can lead to a point mutation
if not repaired by the DNA repair enzyme
s such as uracil glycosylase, which cleaves a uracil in DNA.
Cytosine can also be methylated
by an enzyme called DNA methyltransferase
or be methylated and hydroxylated
to make 5-hydroxymethylcytosine
. The difference in rates of deamination of cytosine and 5-methylcytosine (to uracil and thymine
) forms the basis of bisulfite sequencing
When found third in a codon
, cytosine is synonymous with uracil
, as they are interchangeable as the third base.
When found as the second base in a codon, the third is always interchangeable. For example, UCU, UCC, UCA and UCG are all serine
, regardless of the third base.
Active enzymatic deamination of cytosine or 5-methylcytosine by the APOBEC
family of cytosine deaminases could have both beneficial and detrimental implications on various cellular processes as well as on organismal evolution.
The implications of deamination on 5-hydroxymethylcytosine, on the other hand, remains less understood.
Cytosine has not been found in meteorites, which suggests the first strands of RNA and DNA had to look elsewhere to obtain this building block. Cytosine likely formed within some meteorite parent bodies, however did not persist within these bodies due to an effective deamination
reaction into uracil
External links and citations
Cytosine MS Spectrum